Technical Field
The present invention relates to a fixing exit guide plate for use in a fixing device, a fixing device, and an image forming apparatus using the same.
Description of Related Art
A fixing device includes a fixing member and a pressure member. Both of the members have contact with each other to form a fixing nip. A recording sheet on which a toner image is transferred is fed to the fixing member, and the toner image is thermally fixed on the recording sheet by heating while being pressed onto a periphery of the fixing member by the pressure member at the fixing nip.
The fixing member and the pressure member have a temperature difference. Due to such a temperature difference, the front and the back of the recording sheet (front on fixing member side and back on pressure member side) also have a temperature difference when the recording sheet passes through the fixing nip between the fixing member and the pressure member in the thermal fixing. More specifically, the temperature of the recording sheet, which has contact with the fixing member, is higher than that of the back of recording sheet, which has contact with the pressure member.
As a result, after the recording sheet passes through the fixing nip, much moisture contained in the recording sheet evaporates from the front of the recording sheet. The amount of moisture to be moved to the front of the recording sheet is increased. The amount of moisture is increased in the front of the recording sheet to be larger than that of the back of the recording sheet. As a result, the amount of extension is increased in the front of the recording sheet to be larger than that of the back of the recording sheet, causing curl of the recording sheet toward the back (hereinafter referred to as back curl).
A fixing device including a fixing member having a reduced heat capacity to increase a temperature increase speed has been recently developed in view of energy saving and a high warming up speed. Such a fixing device has a large temperature difference between the fixing member and the pressure member and also has a large temperature difference between the front and the back of the recording sheet in fixing since the recording sheet passes between the fixing member and the pressure member before the pressure member is warmed up. For this reason, large back curl of the recording sheet is generated in the fixing device. When the recording sheet having such large back curl is ejected, a predetermined number of the recording sheets cannot be stacked on a paper ejection tray, or the recording sheet cannot be appropriately stacked on the paper ejection tray.
To solve such a problem, a technique that corrects such curl of the recording sheet by pressing the recording sheet onto a guide plate has been proposed.
For example, in an image forming apparatus described in Patent Literature 1 (Japanese Laid-Open Patent No. 2001-48399), a first conveyance roller is disposed downstream of a fixing unit, and a second conveyance roller is disposed downstream of the first conveyance roller. A guide plate that curves a sheet in the direction opposite to the curl direction of the sheet is provided in a conveyance path from the first to second conveyance rollers. The conveyance speed of the sheet by the second conveyance roller is set to be faster than the conveyance speed of the sheet by the first conveyance roller. The sheet is thereby fed while being pressed onto the guide plate that curves the sheet in the direction opposite to the curl direction of the sheet, and thus, the curl of the sheet is corrected.
An image forming apparatus described in Patent Literature 2 (Japanese Laid-Open Patent No. H09-188456) includes a pair of rollers provided downstream of a fixing device, and a guide plate provided just after the rollers. The guide plate conveys a sheet ejected from the rollers while curving the sheet toward the opposite side, so that the curl of the sheet is corrected by curving the sheet in the direction opposite to the direction of the curl generated in the fixing device.
In a fixing device described in Patent Literature 3 (Japanese Laid-Open Patent No. 2012-91891, a guide plate is movably disposed between a fixing nip and an ejection nip to have contact with the back of a sheet, so as to curve the sheet in the direction opposite to direction of the curl generated in a fixing unit, and change a contact force by the strength of the sheet.